PSI - Issue 5

Sahand Pourhassan et al. / Procedia Structural Integrity 5 (2017) 1355–1362 Sahand Pourhassan et al./ Structural Integrity Procedia 00 (2017) 000 – 000

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correlation technique, which allows the determination of the object contour, as well as the surface displacements with high local resolution. In order to achieve a good correlation, this method uses a speckle pattern applied to the object surface and tracks the grey value pattern in small neighbourhoods called subsets during deformation. After acquisition, these images are digitized and stored for analysis. Two subsets are chosen respectively for the reference and deformed images for correlation. This technique is used for 2D and 3D deformation, strain measurement on components, for simultaneous detection of longitudinal and transverse strains with any load, such as tensile, compression, bending and torsion or a combination of different loads [2], [3]. Considering factors like cost efficiency and ease of use when compared to speckle interferometery, or its great precision when compared to traditional sensing methods, DIC emerges as a robust tool for both indoor and outdoor applications [4]. The method is able to precisely determine 2D and 3D strain fields and therefore it’s possible to assess various material behaviors including metals, ceramics and plastics, bio-materials, polymers, rocks, glass, foams, sands, soils, clay and shape memory alloys. The fact that no physical sensor is involved in the process makes DIC even more flexible since it enables measuring almost any type of deformation in time and space, giving access to information about strain gradients and their variations in time. Therefore, DIC technique was applied on current work to characterize the material behaviour of 2024-T3 ALCLAD and 2124-T851 aluminum alloys and to propose and validate modified geometries compatible for different testing machines at high strain rates. The goal of this work is to reduce the bias that may cause from using totally geometries w hen analyzing strain rate dependency of materials using different testing machines and acquisition system. 2.1. Materials and specimen geometries The mechanical properties of two different metallic materials were studied, 2024-T3 ALCLAD and 2124-T851 aluminium alloys, using 2D and 3D DIC measurement techniques. Apart from the flat and cylindrical standard geometries defined by ASTM [5], two modified specimen geometries were also included in this work. The designed flat specimen and its corresponding gripping system, Fig 1 and Fig. 3 , is similar to the one proposed by Ledford et al. [6] Likewise, cylindrical specimen was adopted to be able to fit into other testing machines, Fig.2. To confirm the geometries series of static tensile tests (10 -3 s -1 ) were carried out using MTS 810 servo hydraulic testing system. As can be seen in the figs below, both flat and cylindrical specimen geometries were divided into standard and non-standard categories. 2. Materials and Methods

Fig. 1. (a) Defined by ASTM E 8M – 04 standard (additional 6mm holes in grip section are for alignment purposes) (b) Non standard specimen geometry.

Fig. 2. (a) - Defined by ASTM E 8M – 04 standard, (b) Non-standard specimen geometry.